Lasers which produce outputs having stable frequencies are widely accepted as reference signal sources used for many scientific, commercial, industrial and military applications. For example, such lasers may be used in a broad array of precision measurements as well as communication systems and other systems which require precise timing. It is desirable for lasers used in such applications to produce optical outputs having a very narrow linewidth and, ideally, center frequencies which exhibit long-term stability. For environmentally demanding applications, such as space-based applications, low power, compact systems are also desirable.
A number of laser control techniques have demonstrated either good center frequency stability or a narrow linewidth. Linewidth, as such term is used herein and in the art, refers to the overall spectral content of the laser output (i.e., frequency spectrum) rather than any resolution of the laser output (i.e., lines/millimeter).
The generally known Pound-Drever-Hall (PDH) locking scheme, for example, provides a very narrow linewidth laser output by matching the output to a very sharp resonant mode of a high finesse optical cavity. The PDH laser system facilities locking of the laser output to an independently selected resonant cavity frequency through feedback control. The PDH feedback control corrects for mechanical variations in both the optical cavity length and laser cavity length due to, for example, temperature based or acousto-mechanical displacements of the mirrors located at the laser and cavity ends. To achieve a stable lock, it is further desirable for the optical resonator of the PDH system to possesses a high finesse, which exists when the reflectivities of the mirrors comprising the optical resonator are very high (i.e. low optical loss from the resonator). Mathematically the finesse is simply the free spectral range (FSR) of the resonator (i.e. the frequency difference between adjacent axial modes) divided by its full-width at half-maximum frequency response (spectral linewidth). Lasers with sub-Hertz relative linewidths have been reported by PDH locking a laser to a high finesse optical resonator.
Prior art solutions for achieving the long-term frequency stability of laser systems have included the use of atomic or molecular resonators as frequency references. Prior art systems and methods most often accomplish this by locking the laser frequency to an atomic transition using absorption information obtained by passing the laser light through a gas cell. The advantage of locking a laser to atomic transitions is that atomic transitions are highly stable frequency references. However, the need for a gas cell and the corresponding limit on achievable linewidth using practical gas pressures and temperatures are disadvantages of these laser systems for environmentally demanding and high reliability applications, such as spaceflight. In addition, this approach is limited to lasers with frequencies near those of molecules that can be practically utilized and are therefore not tunable. This limits the laser to be controlled to a frequency corresponding to a strong atomic transition in the gas. Furthermore gas cells are not rugged and typically require a large apparatus to accomplish laser control.
Other prior art techniques for stabilizing the long-term frequency of a laser rely on the locking of an optical cavity to a radio frequency (RF) oscillator by locking a single phase modulated laser and one side band to the n-th and n-th+1 order of the cavity. This limits the free spectral range of the cavity to be such that a phase modulator can be built at that frequency. This method of laser locking suffers at least three drawbacks. First, the method works well for long cavities with relatively small FSRs but poorly for compact systems with a FSR&gt;100 MHz in which noise from the external reference limits the stability of the cavity length. Second, noise associated with laser phase modulators, especially at higher frequencies (&gt;100 MHz), limits the performance of this technique. Third, this laser locking system suffers from the same disadvantage as all others in that it controls some aspect of the laser itself, such as the cavity length or other intercavity element, to set its frequency. Consequently, the two frequencies used to lock the cavity, the laser output frequency and that same laser's RF sideband are themselves phase-locked and, therefore, must be insensitive to certain changes in the optical cavity length. Finally, RF laser locking techniques require a phase modulator with a high bandwidth, thereby implying high power and an inefficient implementation.
Therefore, the need exists for apparatus and methods of achieving a compact laser frequency control system, which consumes minimal power, has an optical laser output with a narrow linewidth and is capable of being designed with almost any output wavelength. Furthermore, methods and apparatus for a compact laser frequency stabilization system in which the long term laser output frequency stability is controlled independently of the linewidth stabilization apparatus is desirable.
Accordingly, it is an object of the invention to provide a laser control system which provides a narrow output laser linewidth and a temporally stable laser center frequency.
It is a further object of the invention to provide a stable reference oscillator having a narrow frequency spectrum and a temporally stable center frequency.